They call it a particle, but at that lifetime and production rate it's better called a resonance state. They suggest the particle might be a pion and a J/ψ meson that slow down for a brief chat when they get close to each other. That's even less impressive than putting two alpha particles next to each other and calling it Beryllium 8.

Felgraf:But you just... you just said "It could be a thing which we've never seen before! It could also be this other thing. But probably not, because we've never seen that second thing before."

It can't be the second thing because of something called the Pauli exclusion principle which states that certain particles can't inhabit the same quantum state in the same place at the same time. It's a bit sloppily written, but it's accurate.